The invention relates to seismic profile systems. More particularly, the invention relates to a surface seismic profile system and method using vertical sensor arrays.
Various methods are known for obtaining seismic profiles of a desired geographical location on land. One of the known methods is the vertical seismic profile (VSP), which is illustrated in FIG. 1. The VSP method requires the existence of a bore hole 1 drilled to a depth "D" below the surface 3. Bore hole depths vary widely, but the bore hole is typically on the order of several thousand or more feet deep.
A plurality of sensors 5 are placed at vertically spaced locations within the bore hole. The sensors may be geophones secured within the bore hole, or may be hydrophones in the event the bore hole is filled with a liquid. A source 7 of seismic energy is placed on the surface at various distances and angular positions with respect to the bore hole, and the signals received by the sensors are collected by a monitor 9.
The important signals from the standpoint of obtaining an accurate seismic profile are the "primary" reflection signals (shown here as P1-P4) which are reflected off a boundary 11 between different subsurface layers 10 and 12 and are received by the sensors as upgoing waves. Direct signals from the source (e.g., D1) are also received by the sensors and can be used for timing purposes. A potential problem is created, however, by those waves (e.g., R1) which are reflected twice; once off the boundary 11 and again off the surface 3. These multiply reflected signals are received as downgoing waves and are 180 degrees out of phase with the primary signals, and thus potentially cause destructive interference with the primary signals known as "ghosting". However, as explained, for instance, in U.S. Pat. No. 4,958,328 to Stubblefield, the vertical arrangement of the sensors allows upgoing and downgoing waves to be distinguished during processing of the data. Thus, the deleterious effects of the downgoing waves can be reduced.
The VSP method is, therefore, well suited for obtaining seismic profiles in the vicinity of an existing bore hole. It is, however, severely limited in the horizontal direction because its effectiveness decreases rapidly as the source is moved away from the bore hole. It is also limited in that it requires the existence of a bore hole in the first place. If no bore hole exists in the vicinity to be surveyed, it would most likely not be cost-effective to drill one simply to allow the VSP method to be utilized.
An application of VSP is also known in the marine environment. In water, the VSP method can be used to advantage because the vertically stacked sensors are no longer limited to a bore hole. As shown in FIG. 2, two sensors 5 can be suspended vertically in the water from a buoy 13, which includes a transmitter 13a for relaying data to a recorder (not shown). A boat 14 tows a source 7 underwater in the vicinity of the sensors. Primary signal P1 is reflected off the boundary 11 between layers 10 and 12 under the sea floor 15 and is received by the sensors 5 as an upgoing wave. Multiply reflected signal R1, which has been reflected off the boundary 11 and the air/water boundary 19, is received by the sensors as a downgoing wave, generating what is known as a source ghost. Multiply reflected signal R1 is an example of only one multiply reflected signal which may occur; many other multiple reflections occur off the air/water boundary 19, the sea floor 15, and the boundary 11. All of these multiply reflected signals interfere with the primary reflected signals, and thus reduce the accuracy of the data obtained by the survey.
Another seismic profile method is known which allows greater flexibility on land in the horizontal direction. The horizontal seismic profile (HSP) method, illustrated in FIG. 3, employs an array of sensors 5 placed on the surface 3 at predetermined locations with respect to a source 7. Primary signals (P.sub.1, P.sub.2, P.sub.3) are reflected off the boundary 11 and are detected by the sensors 5. The source and the sensors can be moved horizontally relatively easily to increase the area of the survey. Thus, the HSP method permits a widely spaced profile. However, the HSP method cannot easily distinguish between primary reflections and multiply reflected waves, because all signals received by the sensors are upgoing. For instance, it is impossible to distinguish between multiply reflected signal R1 and primary reflected signal P3 using the HSP method. Thus, it is difficult to obtain accurate data using this prior art method.
The problems discussed above are further aggravated by the geology of "transition zones", that is, those areas which are between the open water and land. Typically, transition zones are characterized by a relatively shallow layer of water over a layer of mud with harder composite or rock layers underneath (see FIG. 4). The air/water boundary 19, the water/mud boundary 23, and the mud/rock boundary 24 are all efficient reflectors. As a result, the desired data are compromised by multiply reflected waves such as R2 which are reflected off the boundary 11, the air/water boundary, the water/mud boundary, and are then received by the sensors. Since this is an upgoing wave, it is virtually impossible to distinguish this wave from an upgoing primary reflection signal.
Transition zones pose a number of problems which are unique to this geology and are not encountered in typical land and marine applications. As explained above, one of these problems is the existence of multiple efficient reflective layers which cause multiple reflections and thus interfere with the primary reflected wave. In addition, wind and surf noise are problems in transition zones. Further, the existence of a lossy medium (mud) in the near surface results in attenuation of high frequencies. Also, only a small number of sensors are typically available due to logistical problems in deployment.
Despite the importance of transition zones to the oil and gas industry, the previously known methods of seismic profiling encounter difficulties in these regions as a result of poor signal to noise ratio and poor resolution of the seismic data.